1. Field of the Invention
This invention pertains to the field of sintered ceramics, particularly magnesium aluminum spinel, and to a process for preparing sintered ceramic articles from ceramic powders.
2. Description of Related Art
Sintering is defined as the act of consolidating powder into a dense shape. The powder being sintered must additionally not melt to a great extent, some melting of secondary phases in the powder, or surface melting is allowed under this definition. If the material completely melts, the process is referred to as fusion casting. Sintering, both presureless and with pressure, or hot pressing, requires solid, liquid or gas material transport to consolidate an aggregate of loose powder particles into a dense shape. In the case of porcelains and clay products, secondary phases do melt and “glue” the primary solid particles together with a glassy phase. These types of systems were the first to be used due to their ease of sintering. However, advanced ceramics do not have these intrinsic sintering aids and they must therefore, be added. For small samples, the powdered sintering aids are mixed with the powder to be sintered with a mortar and pestle. In larger samples, mixing is accomplished by ball milling, attritor milling, high shear wet milling, and variations or combinations of these methods.
Spinel is defined as a crystalline structure of the type AB2O4 where A is a 2+ cation occupying tetrahedral lattice site in an oxygen cubic close packed structure and B is a 3+ cation occupying octahedral lettice site. In a preferred embodiment, spinel is MgAl2O4 consisting of an oxide of magnesium and aluminum. Spinel powder can be prepared by wet chemistry, solid state diffusion of oxides or calcination. Spinel powder particles consist of crystallites which are less than 500 nm in size that can also be agglomerated into larger sizes varying from 500 nm to 100 μm, more typically 1-50 μm.
Spinel is important because it is strong and transparent from visible to 5.5 μm wavelength. Its mechanical properties are several times greater than that of glass and make it a leading candidate for use as a transparent armor and window material. Commercially, it can be used as a stronger and thinner window for many applications including lap top computers, cell phones, automotive glassing and headlamps, aerospace windshields, and industrial blast shields.
Dense, transparent spinel articles are not currently available from a commercial source although there are companies currently trying to develop a viable manufacturing process. Since there is no viable manufacturing process, the cost of spinel products is so high that even the military avoids its use.
Difficult to sinter materials, such as spinel, are typically mixed with a sintering aid or a secondary material that aids in densification. The sintering aids work in a variety of fashions. The sintering aids may liquefy at or somewhat below the primary material's densification temperature thereby promoting liquid phase sintering. Certain sintering aid materials exhibit higher solid-state diffusion coefficients than the primary material's self-diffusion coefficient. The secondary material may conversely have a lower solid-state diffusion coefficient that prevents exaggerated grain growth and promotes grain boundary refinement and pinning. The sintering aid may also simply clean or etch the primary material's surfaces thereby enhancing solid-state diffusion. These are broad examples of the mechanisms by which sintering aids enhance densification. In actual practice, sintering aids may not fit into just one of the categories outlined and the same aid may have different functions in different material systems, or have no effect in other systems.
Sintering aids tend to be solid inorganic particles at room temperature. Sintering aid particles henceforth are defined as comprising crystallites (≦500 nm), crystals (>500 nm), and agglomerates of crystallites and/or crystals. Since the materials to be densified are generally also solid inorganic particles, the two materials must be mixed homogeneously for the sintering aid to be effective. This is accomplished by some form of mechanical mixing. However, due to the nature of particle-particle interactions, the mixture is far from homogeneous. Inhomogeneity in the mixture results in areas that have too much sintering aid and other areas that have little or no sintering aid. This is a major problem in the fabrication of transparent ceramics, electronic ceramics, and in high tech refractory ceramics.
The Sellers et al U.S. Pat. No. 3,768,990 discloses an optical element having transparency in the visible and infrared wave lengths that is made by heating at an elevated temperature a composition having sub-micron particle size of magnesium oxide and aluminum oxide having uniformly mixed therethrough 0.2-4% by weight of powdered LiF. It is believed that optical and mechanical properties of the Seller's optical element are negatively impacted by the inhomogeneous presence of substantial amount of LiF. This leads to microstructural regions that are highly porous and other microstructural regions that exhibit exaggerated grain growth, all of which lead to inferior optical and mechanical properties. This has prevented the use of spinel in practical applications since the Seller's patent issued in 1973. Furthermore, it is believed that the atomic concentrations of lithium and fluorine will be greater than about 1000 ppm and 100 ppm, respectively due to the fact that LiF is well known to react with alumina, which Seller's uses as a starting powder.